This invention relates to antennas, and more particularly to satellites with dual-polarization antennas including a separate feed for each polarization.
Communication satellites are in widespread use for communicating data, video and other forms of information between widely spaced locations on the earth's surface. It is well known that communication satellites are expensive, and that they have a lifetime which is limited by consumption of expendables, notably consumption of propellant which is used for attitude control and for North-South stationkeeping. In order to provide as much propellant as possible at the beginning of a spacecraft's life, the weight of every portion of the spacecraft is scrutinized, and costly tradeoffs are made to save weight to allow on-loading of additional propellant to extend the life of the satellite. The value of a single month of additional operation of a satellite can be millions of dollars, so a weight saving of even a few pounds, for which propellant can be substituted, may result in tens of millions of dollars of savings.
Among the larger structures on the spacecraft are the solar panels, which require a relatively large surface facing the sun in order to intercept sufficient energy to generate electricity for the spacecraft's operation, and the transmitting and receiving antennas.
The antennas are transducers between transmission lines and free space. A general rule in antenna design is that, in order to "focus" the available energy to be transmitted into a narrow beam, a relatively large "aperture" is necessary. The aperture may be provided by a broadside array, a longitudinal array, an actual radiating aperture such as a horn, or by a reflector antenna which, in a receive mode, receives a collimated beam of energy and focuses the energy into a converging beam directed toward a feed antenna, or which, in a transmit mode, focuses the diverging energy from a feed antenna into a collimated beam.
Those skilled in the art know that antennas are reciprocal devices, in which the transmitting and receiving characteristics are equivalent. Generally, antenna operation is referred to in terms of either transmission or reception, with the other mode being understood therefrom.
For various reasons relating to reliability, light weight and cost, many current communication satellites employ "frequency re-use" communications systems. Such a system is described, for example, in U.S. patent application Ser. No. 07/772,207, filed Oct. 7, 1991 in the name of Wolkstein. In a frequency re-use system, independent signals are transmitted from a earth station over a plurality of band limited "channels" which partially overlap in frequency. At the transmitting earth station, mutually adjacent channels are cross-polarized. In this context, cross-polarization means that the signals of a particular channel are transmitted with a particular first polarization, while the signals of the two adjacent channels are transmitted at a second polarization orthogonal to the first. Ordinarily, each of the two orthogonal polarizations are two linear polarizations, which may be referred to as "vertical" and "horizontal", although, as known, precipitation causes rotation of the polarization. In principle, the two orthogonal channels could be right and left circular polarizations, but linear vertical and horizontal are more easily controlled. At the satellite, the vertically and horizontally polarized signals are separated by polarization-sensitive antennas and applied to separate transmission lines. This has the result which, in each channel, tends to suppress the signals relating to the two adjacent channels. Thus, even though the frequencies of the signals in each channel partially overlap, the overlapping frequency adjacent-channel signals are suppressed, which tends to reduce interchannel interference.
In the satellite, the received signals from the vertically and horizontally polarized antennas are converted to a different frequency range, filtered, and amplified by an amplifier within each channel, to produce independent signals in adjacent channels with partially overlapping frequencies within the converted frequency range, which independent signals are then combined or demultiplexed, and every other (or alternate) channels are applied to one polarization of a dual polarization antenna for retransmission back to the earth. As in the case of the receiving or uplink antenna, the transmitting or downlink antenna tends to maintain a degree of isolation between each channel and its immediate neighbors.
A prior art antenna which has been used for communication satellites includes a first reflector made up of mutually parallel, "vertically" polarized conductors lying along a surface having the shape of a parabola of revolution, and having a focus at which a vertically polarized feed antenna structure is located. Vertically polarized signals are reflected by the first reflector acting as a parabolic reflector, to collimate diverging signals radiated by the feed antenna to form a collimated beam which is directed toward the ground station, and for receiving collimated signals from the ground station and focusing the collimated signals onto the feed antenna. Horizontally-polarized signals, however, pass unimpeded through the vertically polarized conductive elements of the first reflector. A second reflector, located immediately before or immediately after the first reflector, consists of a plurality of mutually parallel, "horizontally" polarized conductive elements, forming a second parabolic reflector having a focal point at a second location different from that of the first focal point. A horizontally polarized feed antenna structure is located at the second focal point.
The above-described prior art antenna requires two separate parabolic reflectors, each formed from a elongated conductive grid, and each with a different focal point. The fabrication of the supports which lie between the two reflectors is difficult, and its presence tends to distort the radiation pattern of the rearmost reflector.
The weight demands on spacecraft militate against large antennas in favor of small antennas, which tend to require greater available transmitter power to achieve the desired carrier-to-noise (C/N) ratio, which in turn tends to require larger solar panels to energize more powerful amplifiers. As an alternative, smaller antennas can be used to achieve a given gain and C/N, if a higher operating frequency is used.
The demands for improved and lower-cost communications have driven communication satellites toward higher transmitted power and longer life. The long life and reliability considerations tend to favor use of solid-state amplifiers, while the high power and high frequency considerations favor the use of travelling-wave tube amplifiers. A way to achieve high power by paralleling solid state amplifiers is described in U.S. Pat. No. 4,641,106, issued Feb. 3, 1987 in the name of Belohoubek et al. Such schemes may be difficult to implement and may not achieve as much output power as a single travelling-wave tube. Another paralleling scheme is described in U.S. Pat. No. 5,103,233, issued Apr. 7, 1992 in the name of Gallagher et al. In the Gallagher et al scheme, an active array antenna includes radiating elements (radiators) on a radiating face of the antenna. Each of the antenna elements is driven by an amplifier of a transmit-receive module in a transmit mode, and, in a receive mode, drives a low-noise amplifier of the module. The phase distribution of the array is established in part by the distribution of an interior feed antenna which radiates to and from a second set of antenna elements on the interior of the array. Phase shifters associated with each transmit-receive module divert or steer the beam relative to broadside. This system may be difficult to implement in a lightweight system.